Interest in the recovery of carbon dioxide (hereinafter “CO2”) from various CO2 containing gas mixtures has increased due to a variety of factors including the merchant CO2 market, enhanced oil recovery (hereinafter “EOR”) and greenhouse gas emissions reduction. The currently available systems for recovering high purity CO2 use a variety of generic and proprietary physical and chemical solvents. Accordingly, the processes utilized for this recovery require a large investment due to equipment costs and also high regeneration energy requirements.
Carbon dioxide containing gas mixtures are produced as waste streams during the production of hydrogen gas from hydrocarbon streams using standard steam hydrocarbon reforming processes (hereinafter “SHR”). The most preferred of the SHR processes involves the production of hydrogen gas from hydrocarbon streams using steam methane reforming (hereinafter “SMR” processes since methane has a higher proportion of hydrogen than other hydrocarbons. More specifically with regard to general SMR processes, a hydrocarbon feed gas is fed into a SMR device where the methane in the feed gas reacts with steam at high temperatures (from about 700° C. to about 1100° C.) in the presence of a metal-based catalyst to produce a mixture of carbon monoxide and hydrogen. The hydrogen yield of this mixture is increased by passing the resulting mixture through a water gas shift reactor which promotes the conversion of carbon monoxide and water into more hydrogen and accordingly a stream that is rich in hydrogen but also contains to a lesser degree carbon dioxide, methane and carbon monoxide. Such units typically operate at a temperature from about 200° C. to about 500° C. In some cases the stream from the SHR will be at a higher temperature so optionally the stream may first be cooled with a heat exchanger before being passed through the water gas shift. The hydrogen rich stream produced is then passed through a H2 pressure swing adsorption unit (hereinafter “H2 PSA”) in order to allow for the removal of about 90% or more of the hydrogen present through the use of adsorbents. The removal of the hydrogen results in a waste stream (also commonly referred to as “tail gas”) that is purged from the H2 PSA that contains methane, carbon monoxide, carbon dioxide, water, and any unrecovered hydrogen. This differs from the SHR units, with the difference being that the waste stream or tail gas produced in the SHR units contains alkanes of varying size (CnH2n+2) and water. The desire has been to be able to utilize these waste streams more efficiently as in the past they have simply been burned as make up fuel (added to the natural gas used in the SHR process or SMR process).
Recently, a CO2 cryogenic process unit (hereinafter “CPU”) process was proposed to capture the CO2 during steam methane reforming pressure swing adsorption off gas (by Air Liquide) in WO 2006/054008. In this process, the waste gas from the CPU plant, which normally contains significant amounts of H2, can be recycled back to the SMR plant for additional H2 production credit. The process requires operation at high pressure and cold temperature though. Therefore, while it may be appropriate to use the CO2 CPU process in a very large scale CO2 recovery plant (>1000 TPD), when applying the CO2 CPU process in a small size CO2 recovery plant (typically 100 to 500 TPD merchant CO2 plants), the energy and maintenance costs are considered to be usually high.
Recovery of CO2 from SMR H2 PSA off gas by using an adsorption process has been proposed by the BOC Group in U.S. Pat. No. 4,963,339 and U.S. Pat. No. 5,000,925 wherein the CO2 was removed using a CO2 PSA unit. To produce food grade CO2 from a CO2 lean stream a two stage process was used with the first stage being a CO2 vacuum swing adsorption (hereinafter “VSA”) or PSA process. In the first PSA or VSA stage, a CO2 lean stream containing <50% CO2 was upgraded to a high concentration CO2 mixture (>90% CO2). This high concentration CO2 mixture was then sent to a second stage which was a standard CO2 liquefaction plant. Therefore, a food grade CO2 product was produced. On the other hand, the CO2 VSA/PSA process normally generates another CO2 lean product. Typically this CO2 lean product contains over 35% H2, 20% CH4 and 15% CO with a balance of CO2 which was sent to the reformer as fuel in current designs.
There exists a need to provide a process for recovering high purity gaseous hydrogen and high purity gaseous carbon dioxide from the gas stream produced using steam hydrocarbon reforming, especially steam methane reforming. There also exists a need for a process for treating the waste gas stream obtained from a H2 PSA unit under optimum conditions in order to allow for the recovery of a high quantity of high purity gaseous carbon dioxide.